1. Field of the Invention
The present invention relates to an apparatus and a method for controlling jitter buffering, and more particularly to such an apparatus and a method applicable to, for example, a telecommunications apparatus connected to an IP (Internet Protocol) network.
2. Description of the Background Art
A telecommunications apparatus transmitting and receiving PCM (Pulse Code Modulation) signals such audio or facsimile signals over an IP network needs to control reception jitter buffering in order to absorb or compensate for fluctuation, or jitter, involved in IP packets.
In a conventional solution for controlling reception jitter buffering, a buffer circuit receives and stores PCM data and additional information, carried on an IP packet asynchronously arriving from an IP network, in the order of the sequence number contained in the header of the IP packet, and outputs the PCM data at a predetermined processing interval, e.g. 10 millisecond (ms), in the order of the sequence number.
Between a transmitter and a receiver side dealing with communications on an IP network, it is difficult to perfectly synchronize the basic periods, e.g. 10 ms, of the respective system clocks with each other. Between both sides, a difference in precision could be caused so that the reception jitter buffer may involve under-running, which may cause appropriate PCM data to lack Additionally, when packet loss occurs in an IP network, the sequence number of received packets skips so as to cause some PCM data not to exist in the reception jitter buffer.
In such cases, in order to avoid discontinuity of the linear signals, the received-signal processor in a conventional solution uses a signal inputted in the past to generate a pseudo linear signal, with which a signal is compensated for, or interpolated, to demodulate digital data and control signals.
However, for example, in facsimile communications over an IP network, in order to demodulate a control signal compensated for, or interpolated, with a pseudo linear signal, a signal in the past is used to generate the pseudo linear signal, resulting in the control signal being incorrectly demodulated, which may cause a communication error.
In order to solve such a problem, a solution is used which is disclosed in a publication “Guideline for VoIP-TA/Facsimile Terminals for connecting a Facsimile Terminal to an IP-PBX through a VoIP-TA”, Communications and Information Network Association of Japan, CES-Q006-1, Oct. 19, 2007. This guideline describes a technique related to facsimile communications over an IP network.
In a solution for controlling jitter buffering shown in FIG. 3.10 of the guideline, in order to prevent a modem signal reception error from being caused by under-running in a reception jitter buffer, the reception jitter buffer stores, at the beginning of facsimile communication, a predetermined volume of data, for example, corresponding to 140 ms, prior to starting to output the data therefrom. Thus, the storage of a predetermined volume of data prior to commencing the output of the data from the reception jitter buffer can prevent the reception jitter buffer from being depleted of data, thereby avoiding under-running.
When packet loss compensation means generates, as described above, a pseudo linear signal on the basis of a past input signal in response to an occurrence of under-running to insert the pseudo linear signal, a modem signal is subject to temporal extension to thereby disarrange its phase, thus the disarrangement continuing even after the period of packet loss. Thereby, the modem signal cannot correctly be demodulated, which may cause a reception error.
When a large difference in processing period is caused between the transmitter and receiver sides, or a large fluctuation is caused through the IP network, the possibility in occurrence of the under-running increases.
Even when the technique described in the above-described guideline is applied, the following problems could occur. For example, when a large precision difference in basic period (for example, 10 ms) of the system clocks is caused between the transmitter and receiver sides, or a large fluctuation is caused through the IP network, or facsimile communication continues over a long period of time, the reception jitter buffer may completely be depleted of a certain volume of data stored therein. Thus, the under-running of the reception jitter buffer cannot completely be eliminated, which is problematic.
Additionally, by increasing the storage capacity of the reception jitter buffer capable of storing data, the occurrence of the under-running in the reception jitter buffer can be reduced. However, a transmission delay in the reception jitter buffer increases, which is problematic.
For example, a protocol for facsimile communications provides a response monitoring period in which it is monitored whether or not a response to the control signal transmitted is received from a transmitter side. Therefore, if no response signal is received within the response monitoring period, the facsimile communication is determined as failure.
Furthermore, when the reception jitter buffer stores a small volume of data stored at the start of facsimile communication over an IP network, a large precision difference in basic period, e.g. 10 ms, of the system clocks between the transmitter and receiver sides, a large fluctuation caused through the IP network, or a long duration of facsimile communication may cause the reception jitter buffer to store a larger amount of data, thereby causing a longer delay in output data as time elapsing. In this case, the transmission delay may increase to result in failure of the facsimile communication, which is also problematic.
The above-described problems can be raised in not only facsimile communications but also audio communications over an IP network.